1. Field of the Invention
The present invention relates to a multilayer plastic pipe having improved barrier action against diffusion of the material being transported by the pipe, while maintaining dimensional stability and mechanical strength.
2. Discussion of the Background
Plastic pipes prepared from polyamide resin are known and are used for various applications. In order to perform the particular application desired, the pipes must be inert to the material flowing in them. Additionally, they must be resistant to both high and low temperatures-and mechanical stresses.
Single-layer pipes are not always capable of meeting these necessary requirements. For example, during transportation of aliphatic or aromatic solvents or fuels, they exhibit several problems, such as a lack of barrier action against the medium, dimensional instability and insufficient resistance to mechanical stresses.
Attempts have been made to solve these problems by the use of multilayer pipes (DE-A 35 10 395; 37 15 251; 38 21 723; 38 27 092; 40 01 125; and 40 01 126). However, in practice, while these multilayer pipes can solve these problems individually, the overall set of properties displayed by the pipes is still unsatisfactory.
German Patent 38 27 092 describes a multilayer pipe which has, from the inside outward, layers of polyamide, polyvinyl alcohol, polyamide and polyester. The polyester is used only as a thin outer layer, in order to achieve a higher resistance to short-term heat deformation. However, it is well known in the field of polymer composites that the majority of polymers, such as polyamides and polyesters, are incompatible with one another. Thus, the typical result of most multilayer composites is that the laminate layers show no adhesion to one another. An intimate bond between the individual polymer layers is absolutely essential for industrial uses, in order to avoid delamination.
Multilayer composites of particular polyamide mixtures and various other thermoplastics, such as, for example, polyethylene terephthalate, are known from EP-A-287 839. Conventionally, in order to achieve necessary adhesion, an adhesion promoter is introduced between the laminate layers. Examples of such adhesion promoters include functionalized polyolefins, functionalized ethylene/vinyl acetate copolymers, ethylene/acrylate copolymers, ionomers, polyalkylene oxide/polyester block copolymers, derivatives of carboxymethyl cellulose and blends of these polymers with polyolefins.
It has now been found that these adhesion promoters do not give a long-term intimate bond, especially in a polyamide/polyester system. Because the adhesion promoters are not sufficiently resistant to thermal deformation and solvents, the adhesion which is achieved is usually lost under conditions of heating or under the action of solvents. Additionally, these composites fail when exposed to shear forces due to cold flow of the adhesion promoter.